What is a thermodynamic cycle?
They’re seen everywhere from power plants to the refrigerator in your home. A thermodynamic cycle is a series of processes that transfers energy to and out of a system. In a refrigeration cycle, the refrigerant transfers energy from inside a building and drops it off outside. Cycles can be classified as power, refrigeration, gas, vapor, closed, and open. The refrigerant in the aforementioned closed vapor cycle is known as the working fluid, and its job is to take energy from one place and drop it off in another. It takes thermal energy from inside one’s home and drops it off outside.
Power and refrigeration cycles
Power cycles produce energy while refrigeration cycles move it around. Of course, energy is not created nor destroyed, so they both take in energy and put some out. The main difference is that power cycles take in energy in one form and use it to produce energy in another. Wind turbines take the kinetic energy from wind, uses it to turn a shaft connected to a generator, and produces electric energy. Internal combustion engines turn the chemical potential energy of the fuel, combust it to produce thermal energy, and uses that energy to push the pistons down, thus turning the axle and turning the wheels. Here, chemical potential energy is used to produce mechanical energy. Refrigeration cycles will take the thermal energy in one area and through various steps will evacuate it to an area of higher temperature. The house is the heat source while the area outside is the heat sink. Refrigeration cycles move energy from cooler heat sources to hotter heat sinks. Since heat naturally travels from higher to lower temperatures, it takes some clever engineering to do this.
Gas and vapor cycles
These two cycles are differentiated by their working fluid. In gas cycles, the working fluid remains as a gas throughout the whole cycle. In vapor cycles, the working fluid is a vapor during one part of the cycle and as a liquid in another part. Refrigeration is a vapor cycle since the refrigerant appears as a liquid in the condenser and pump, while it appears as a vapor in the compressor and evaporator. In the Brayton cycle for gas turbines, no liquid is present and gas is the only working fluid.
Closed and open
Closed cycles return the working fluid to its initial state, or recirculates it. In open cycles, the working fluid is renewed at the end of each cycle. Refrigeration cycles are closed, since the same refrigerant is always in the cycle. The Otto cycle for internal combustion engines is an open system, since fresh air and gas vapor enters the engine at the beginning of every cycle. There is no recirculation in open cycles.